Higher memory (RAM) support

[darvin]

How hard would it be to extend emulator to support 640k+ memory? Which mode would be the easiest to code correctly target?

[dbalsom]

Two options I am considering are implementing the Lo-tech EMS board https://www.lo-tech.co.uk/wiki/Lo-tech_2MB_EMS_Board
or something more period-accurate like the Intel Above Board, if I can find documentation

[darvin]

do you think its necessity? is it possible to circumvent it and just increate the amount of the exposed memory, allowing operating system to handle it however it wishes? Obviously, total majority of applications will fail to notice the exposed memory: 640k barrier is hardcoded in DOS. However those applications that are informed about presence of additional memory over first 640k will be able to access it.

is there a.reasoning behind true to metal implementation of the memory?

[dbalsom]

If you use GLaBIOS you can use 736K of memory with MartyPC, but you bump up against the memory map of the CGA card, and above that there could be other devices mapped and option ROMs, so it is not practical to treat the address space there as RAM.

Typically, to access more than conventional memory in DOS, there were two standards, EMS and XMS, which for an 8088 machine required appropriate hardware and drivers.

[google0101-ryan]

Because no program can possibly use this extra RAM. This is before the times of the a20 line, so programs had a hardcoded limit of 0xFFFFF, and addresses higher than that wrap around to 0. So it is not possible to "just increase the amount of exposed memory." However, this is remedied with the 80286 and newer.

At least, that's how I see it

[dbalsom]

I've finally added support for an EMS card in
5948f12

Now you can add 2MB of EMS memory to your PC/XT